The present invention is directed to an improved method for positioning cutting edges of a pair of circular cutting blades of a longitudinal cutting or slitting device for web paper or the like.
A longitudinal slitting apparatus and a method of positioning the coacting cutting edges of a pair of circular cutting blades is disclosed in German Letters Patent No. 21 42 117. As disclosed, a carriage with a circular cutting blade or knife is shifted by a conveyor arrangement into predetermined positions which correspond to the desired width for sub-webs which are to be slit or cut from a wide paper roll. As long as the position of the cutting edges or surfaces of the circular knives remain invariable relative to the carriage, this positioning can be arbitrarily repeated and the widths of the sub-webs can be set with a tolerance given by the working precision of the conveyor means or device.
Since the stress and the wear due to the paper web passing through the coacting cutting edges with a speed up to 2500 meters per minute is considerable, the circular knives or blades must be reground from time-to-time. This is particularly true for the driven or so-called bottom blade which is designed pot-like as a cylindrical member with the end surface of the member coacting with a relatively flat so-called top cutter. During regrinding, this end surface is ground off so that the pot or cylindrical member becomes smaller along the axial direction of the blade. The amount of reduction in the axial length for each sharpening or grinding operation is in the range of 1 to 2 millimeters.
When positioning is restricted to bringing the carriages to a specific location, the position of the cutting edges relative to the carriages will be changed. Dislocation enters into the width of the sub-webs produced and thus deviates from the rated dimension by a corresponding amount.
This is inadmissible in many instances. It is known to compensate for the change in the position of the cutting edges due to grinding with shims. This method wastes time and is subject to error because the needed shims must be determined by means of measurements which must be undertaken with each pair of blades and errors due to tolerances will still remain.